在尝试定位iOS（在MacBook Air和GitHub Actions运行器上）时，我遇到了同样的问题，尽管我对Apple的生态系统不够熟悉，无法提出适当的解决方案，但这里还是有一些关于此问题的想法。最初的命令行来自cpprestsdk中的CMake，但是一旦我将其归结为基本要素，这里便是一个简短的副本。
- 创建
cmath-bug.cpp其中只有一行的文件：
#include <cmath>
- 运行（某些参数前面的换行符是为了方便阅读，请将其删除）
clang -v -x c++ -target arm64-apple-ios13.2 -fcolor-diagnostics -std=c++11 -stdlib=libc++
-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk
-isystem /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include
-c cmath-bug.cpp
当我运行它时，我会遇到许多面临相同问题的人：
Apple clang version 11.0.0 (clang-1100.0.33.16)
Target: arm64-apple-ios13.2
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" -cc1 -triple arm64-apple-ios13.2.0 -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -Werror=implicit-function-declaration -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name cmath-bug.cpp -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -fno-strict-return -masm-verbose -munwind-tables -target-sdk-version=13.2 -target-cpu cyclone -target-feature +fp-armv8 -target-feature +neon -target-feature +crypto -target-feature +zcm -target-feature +zcz -target-feature +sha2 -target-feature +aes -target-abi darwinpcs -fallow-half-arguments-and-returns -dwarf-column-info -debugger-tuning=lldb -ggnu-pubnames -target-linker-version 530 -v -coverage-notes-file /Users/myuser/Projects/C++/cmath-bug.gcno -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/11.0.0 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk -isystem /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include -stdlib=libc++ -internal-isystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1 -Wno-framework-include-private-from-public -Wno-atimport-in-framework-header -Wno-extra-semi-stmt -Wno-quoted-include-in-framework-header -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /Users/myuser/Projects/C++ -ferror-limit 19 -fmessage-length 204 -stack-protector 1 -fstack-check -mdarwin-stkchk-strong-link -fblocks -fencode-extended-block-signature -fregister-global-dtors-with-atexit -fobjc-runtime=ios-13.2.0 -fcxx-exceptions -fexceptions -fmax-type-align=16 -fdiagnostics-show-option -fcolor-diagnostics -o cmath-bug.o -x c++ cmath-bug.cpp
clang -cc1 version 11.0.0 (clang-1100.0.33.16) default target x86_64-apple-darwin19.0.0
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include/c++/v1"
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/local/include"
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/Library/Frameworks"
ignoring duplicate directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include"
#include "..." search starts here:
#include <...> search starts here:
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/11.0.0/include
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/System/Library/Frameworks (framework directory)
End of search list.
In file included from cmath-bug.cpp:1:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cmath:318:9: error: no member named 'signbit' in the global namespace
using ::signbit;
~~^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cmath:319:9: error: no member named 'fpclassify' in the global namespace
using ::fpclassify;
~~^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cmath:320:9: error: no member named 'isfinite' in the global namespace; did you mean 'finite'?
using ::isfinite;
~~^
我在原始命令行中传递的仅有2个包含目录，它们分别是：
$ ls -alF /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk
lrwxr-xr-x 1 root wheel 12B Dec 17 11:54 /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk@ -> iPhoneOS.sdk
$ ls -alF /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include
total 2160
drwxr-xr-x 169 root wheel 5.3K Dec 17 12:07 ./
drwxr-xr-x 5 root wheel 160B Nov 4 19:22 ../
...
-rw-r--r-- 9 root wheel 32K Nov 4 19:52 math.h
...
但是，这里有趣的是它报告的不存在的包含目录以及最终搜索到的包含目录及其顺序。我的猜测是，Apple Clang的驱动程序会根据某些Apple特定的逻辑插入命令行中未提及的其他目录。
您可以从报告的错误中看到在以下<cmath>位置找到了标题：在标题的/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/cmath第304行中，您可以看到：
#include <__config> // Line 304
#include <math.h> // This one ends up causing troubles
#include <__cxx_version>
从以下事实来看，在同一文件夹中/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/有一个文件math.h提供了必要的定义，例如：
#include <__config>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
#include_next <math.h>
#ifdef __cplusplus
// We support including .h headers inside 'extern "C"' contexts, so switch
// back to C++ linkage before including these C++ headers.
extern "C++" {
#include <type_traits>
#include <limits>
// signbit
#ifdef signbit
template <class _A1>
_LIBCPP_INLINE_VISIBILITY
bool
__libcpp_signbit(_A1 __lcpp_x) _NOEXCEPT
{
return signbit(__lcpp_x);
}
#undef signbit
template <class _A1>
inline _LIBCPP_INLINE_VISIBILITY
typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
signbit(_A1 __lcpp_x) _NOEXCEPT
{
return __libcpp_signbit((typename std::__promote<_A1>::type)__lcpp_x);
}
...
#elif defined(_LIBCPP_MSVCRT)
...
#endif // signbit
的作者<cmath>希望math.h首先包含同一文件夹中的，然后该#include_next <math.h>指令找到系统特定的math.h。然而，事实并非如此。
如果查看搜索目录中的前两个条目：
#include <...> search starts here:
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1
您会看到特定于系统的include目录最终位于通过Clang注入的标准库目录之上，这就是为什么math.h找到特定于系统的目录，而不是与其余标准库标头位于同一文件夹中的原因。可能是这种情况，因为如果我在其他两个目录之前将标准库的include目录显式添加到命令行中-isystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1，问题就会消失并且我可以编译该文件。这不是Clang的驱动程序或此处涉及的其他任何东西自动执行的操作：它通过-internal-system（不确定内部标志的语义是什么）添加了标准库目录，并在系统目录之后添加了它。
现在，如果您查看被忽略目录的列表，则该列表中的第一项是：
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk/usr/include/c++/v1"
其中的结尾c++/v1部分在我的机器上不存在，这让我想知道iPhone SDK安装是否应该c++在路径的现有部分内创建指向/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++目录的符号链接，以使整个工作正常进行。
无论如何，这是我认为正在发生的事情，我想知道是否有人知道如何正确解决此问题？
谢谢！
PS对于上下文：
$ xcode-select -p
/Applications/Xcode.app/Contents/Developer
$ xcrun --show-sdk-path -sdk iphoneos13.2
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk
xcode-select -p匹配其中的Xcode所在？您是否可以将代码更改using std::signbit;为，其他代码也一样？您正在编译为C ++ 11或更高版本吗？